Inrut,
I dont know if the rear stock from a 835 will fit the 500 but forestock will not. The barrel of the 835 is based off of a 10 gauge as where the 500 barrel is based off of a 12 gauge. This leaves the barrel of the 835 larger than the 500.
Here is the contact info for Mossberg...
O.F. Mossberg & Sons, Inc.
7 Grasso Ave.
North Haven, CT 06473
Telephone: (203) 230-5300 (8 a.m. - 4:30 p.m. E.S.T.)
Fax: (203) 230-5420
For Service Inquiries: 800-363-3555